1. Technical Field
The present invention is generally directed to a method and system to accommodate multiple mapping schemes in a single carrier frequency division multiple access (SC-FDMA) system. The present invention is more specifically directed to the use of an orthogonal direct sequence spread spectrum technique to accommodate different mapping schemes, and to a technique to be applied as part of creating a single carrier code-frequency division multiple access (SC-CFDMA) system that accommodates localized and distributed mapping schemes.
2. Related Art
Currently, several wireless communication standards use orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) to achieve high bit rates. In these approaches, a signal is “spread out” and distributed among subcarriers, which send portions of the signal in parallel. The subcarrier frequencies are chosen so that the modulated data streams are orthogonal to each other, such that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. The receiving end reassembles the portions that were sent in parallel. FIG. 1 is a flow chart of transmission and reception within an OFDMA system. OFDM and OFDMA systems suffer from a high peak-to-average power ratio (PAPR), a need for an adaptive or coded scheme to overcome spectral nulls in the channel, and high sensitivity to carrier frequency offset.
SC-FDMA overcomes some of the problems present in OFDM and OFDMA systems by performing a Fourier transform on the signal and then using subcarriers to send it through a serial transmission rather than in parallel. On reception of the transmission, an inverse Fourier transform is performed. FIG. 2 is flow chart of this process. Although SC-FDMA offers a lower PAPR than do OFDM and OFDMA, its effectiveness is limited by the choice of mapping scheme employed. Two approaches exist for SC-FDMA systems to apportion subcarriers among terminals. In localized SC-FDMA (LFDMA), each terminal uses a set of adjacent subcarriers to transmit its symbols. Thus, the bandwidth of a LFDMA transmission is confined to a fraction of the system bandwidth. LFDMA can potentially achieve multi-user diversity in the presence of frequency selective fading if it assigns each user to subcarriers in a portion of the signal band where that user has favorable transmission characteristics. The alternative approach is distributed SC-FDMA, wherein the subcarriers assigned to a terminal are spread over the entire signal band. This approach is robust against frequency selective fading because information is spread across the entire signal band. One realization of distributed SC-FDMA is interleaved FDMA (IFDMA) where occupied subcarriers are equidistant from each other.
FIG. 3 is a comparison of the two mapping schemes. In this figure, three terminals are present, each transmitting symbols on four subcarriers in a system with a total of twelve subcarriers. With LFDMA, terminal 1 uses subcarriers 0, 1, 2, and 3; in the distributed scheme, terminal 1 uses subcarriers 0, 3, 6, and 9.
This current SC-FDMA approach is flawed in certain respects. For example, conventional SC-FDMA cannot efficiently accommodate both distributed and localized mapping schemes for different simultaneous mobile users, or for a relatively stationary user and a highly mobile user, because subcarriers must not overlap.
Accordingly, there is a need for a method and system that accommodates the transmission of localized and distributed mapping schemes so as to take advantage of the strengths of each scheme. There also is a need for a method and system that accommodates a plurality of simultaneous mobile signals and a relatively stationary signal concurrently with a highly mobile signal. It is to these needs and others that the present invention is directed.